We're going to take a pen & pencil drawing and suck it into Photoshop, clean it up, strip the coloured pencil out and recolour it.
Scan in your pencil-coloured image, at a resolution that is about double your final needs. It is better to make your cleanups then shrink the final to it's final size. Eliminates any tiny leftover glitches and preserves detail.
Bring up your Image >> Adjustment >> Levels and adjust it until your outlines are a nice tight black. Doesn't really matter about the rest as long as there is minimal grey and dark tones. You may need to tweak each of the R G B channels individually.
Once you have reached a suitable state of outlinishness, you need to change the Image >> Mode >> to CMYK. This makes it so much easier to extract the black outlines and erase the colours only. You can try the old method of switching to Mode >> Greyscale and then use levels, but I find this methods offers me a better result more often.
Once in CMYK mode, Select >> All (Ctrl A) and choosing each of the C, M & Y channels in turn, hit delete. Choose the K (black) channel and bring up your Image >> Adjust >> Levels dialog box, and play slidermania until you get a final outline and minimal (preferably none) greying anywhere else in the image.
Switch back to Image >> Mode >> RGB.
Returning to the layer palette, you will now see a mostly-white background layer. Double-click this and set it as a layer (by naming it or not as you choose, then clicking OK). Set this layer's Blending Mode to Multiply. This basically turns all the white area transparent, but leave the black as opaque non-see-thru.
Now, if you create layers underneath this layer (by holding down the control key when you click the new layercaicon) you can paint your colours in underneath your black outlines, however you choose.
I make no excuses for my drawing skills, they suk, but this is just to illustrate a point. The red colour is on the bottom layer, with a green-ink'd layer between it and the black outlines. I can't stay inside the lines when I colour in, but you can see a total lack of red or green outside of the lines. You could put photos, textures, patterns, whatever on these layers, and then crop to the black outlines. All sorts of things are possible.
But that's your job, not mine. :)
So get to it, enjoy and have a blast!
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